Numerical study on the topographic influence of the track deflection of Typhoon Megi (2010) during the island-passing period

Abstract:In this study,numerical simulation and diagnostic analysis using model output data are conducted,in order to investigate the mechanism of track deflection of Typhoon Megi(2010) after its landing on Luzon Island.Next,the spectral nudging technique is adopted to reduce the error of the simulated large-scale environmental field.Based on the simulation results of the spectral nudging tests,terrain sensitivity tests are then conducted by varying the terrain height of Luzon Island.The temporal evolution of circulation structure and variable field are diagnosed to investigate the impact mechanism of high terrain on the typhoon track deflection.Finally,the southward movement when the typhoon approaches and lands on the island is displayed.The simulation results indicate that the simulation typhoon track of SNV700,which uses the configuration of nudging synoptic-scale wind component above 700 hPa,can reproduce the southward movement when the typhoon passes by the island and suddenly turns north after leaving the island,by reflecting both the steering role of large-scale environmental flow and the response of lower-level flow to the terrain.Based on the simulation results of the spectral nudging tests,terrain sensitivity tests are then conducted by varying the terrain height of Luzon Island.When the terrain height is reduced to zero,the simulated typhoon track is in correlation with the steering flow.The impact of high terrain leads the typhoon to turn to south(north) when the typhoon approaches(leaves) the high terrain,and the deflection degree increases as the terrain height increases.The results of the terrain sensitivity tests are diagnosed,and backward trajectory analysis of the affected areas is conducted,so as to investigate the temporal evolution of circulation structure and variable field when the typhoon approaches and crosses the island.The results show that the convergence center occurs in both the windward and leeward slopes,thus contributing to the gale center in these areas.In addition,a positive vorticity area occurs in the leeward slope,thereby indicating that secondary depression is induced,which appeals to the typhoon.Meanwhile,the outer and inner circulations of the typhoon are affected by the high terrain.The channeling effect contributes to the gale centers and confluent trajectories between the high terrain and typhoon center.The north(south) low-level jet in the windward(leeward) slope induced by the channeling effect leads the typhoon toward the south(north).